Homework Help:
Equilibrium giving me some trouble

I just got done doing about 20 other problems but I can't figure this one out....

A uniform plank, with a length L of 7.63 m and a weight of 420 N, rests on the ground and against a frictionless roller at the top of a wall of height h = 5.21 m. The plank remains in equilibrium for any value of q= 70° or more, but slips if q < 70. Find the coefficient of static friction between the plank and the ground.